Arrow pushing common mistakes

A common mistake for beginning students is to erroneously write an arrow pointing from the H of the acetic acid to the O atom of the hydroxide anion. This is wrong, because such an arrow would be indicating the H atom movement directly, not electron movement. Other common mistakes in arrow pushing are given in Appendix 10. 

Reversing the direction of one or more arrows during a chemical step is the most common mistake made by students when writing organic reaction mechanisms. Backward arrow pushing usually derives from a student thinking about the movement of atoms, not the movement of electrons. Hence, to avoid this mistake it is important to remember that arrows depict how electrons move, not where atoms move, within or between chemical structures. Further, one can avoid this mistake by remembering that every arrow must start at an electron source (a bond or lone pair) and terminate at an electron sink (an atom that can accept a new bond or lone pair). 

Likely the most common mistake is pushing the arrow backwards. In other words, the arrow is started at a sink and ended at a source. Three examples are given below. The easiest way to avoid this mistake is to remember that the arrow must start from an electron rich region of a molecule. Most important, the arrow always starts with two electrons—namely, lone pairs, o bonds, or tt bonds. Do not use the positive regions of a molecule to start an arrow. The vast majority of the time the arrow will terminate at a center with some positive charge or a center that can accept a lone pair. 

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